1. Field of the Invention
This invention relates to an apparatus for scanning a light beam such as a laser beam. The field to which the present invention is particularly suited is a recording apparatus such as a copying apparatus or a printer which uses a light beam to accomplish image formation.
2. Description of the Prior Art
There has heretofore been utilized a light beam scanning apparatus of the construction as shown in FIG. 1 of the accompanying drawings wherein a rotatable polygonal mirror is fixedly held on a rotary shaft and deflects a light beam.
The construction of FIG. 1 will hereinafter be described briefly. In FIG. 1, reference numeral 1 designates a well-known rotatable polygonal mirror member having side surfaces as mirror surfaces, reference numeral 2 denotes a rotary shaft, and reference numeral 5 designates a rotating device (hereinafter referred to as the motor) as rotary shaft driving means.
In the construction of FIG. 1, the rotary shaft 2 is supported by bearing members 7 and 8 such as ball bearings provided in the immovable portions 17 and 18, respectively, of the casing 6 of the rotating device 5. The motor 5 has a stator 10 mounted on the inner wall of the casing 6 and having a stator coil 9, and a rotor magnet 11 mounted on the rotary shaft 2. By a power being supplied to the stator coil 9, the magnet 11 is rotated with the shaft 2, whereby the polygonal mirror member 1 is rotated and scans a light beam.
In FIG. 1, reference numeral 12 designates a compression spring positioned between an immovable portion 16 in the casing 6 and the bearing member 8. Reference numeral 13 denotes a rotation detecting disk mounted on the rotary shaft, reference numeral 14 designates a rotation detecting sensor, and reference numeral 15 denotes a set screw for fixing the rotatable polygonal mirror member to the rotary shaft.
The use of the light beam scanning apparatus of this type causes the image or the beam to be disturbed by a repeated use and high-speed rotation of such apparatus also causes the image or the beam to be disturbed. The problem that due to such disturbance, the image to be formed is not obtained or the beam is not properly scanned has been difficult to overcome.
Such difficult problem has several basic causes. The main ones of these causes are the surface fall of the polygonal mirror resulting from the difficulty encountered in adjustment of unbalance and the adherence of lubricant to the polygonal mirror which occurs from the bearing members. The surface fall is the irregularity of each mirror surface relative to a certain reference surface when a laser beam emitted from a predetermined position is scanned onto a light-receiving member such as a photosensitive medium by the reflection of the rotatable polygonal mirror. Specifically, as shown in FIG. 2 of the accompanying drawings, a beam emitted from a laser device is reflected by a mirror surface 1a of the rotatable polygonal mirror 1 and reaches a photosensitive medium 3, and in that case, if the mirror surface 1a causes great irregularity as indicated by 1b relative to the reference surface, the position of the beam 4 scanned onto the photosensitive medium 3 differs as indicated by 4b from mirror surface to mirror surface and therefore, the irregularity appears as pitch irregularity of the image or the image disappears.
In FIGS. 2a, 2b and 2c of the accompanying drawings, the deformation of the rotatable polygonal mirror 1 when mounted on the rotary shaft is indicated by broken line.
To mount the well-known rotatable polygonal mirror member 1 having side surfaces as mirror surfaces onto the rotary shaft 2 operatively associated with a rotative driving device, there would occur to mind fixing devices as shown in FIGS. 2a, 2b and 2c. The fixing device shown in FIG. 2a is such that a receiving member 19 having a receiving surface 19a orthogonal to the rotary shaft 2 is fixedly mounted on the rotary shaft 2 and the rotatable polygonal mirror member 1 is fixed to the receiving member 19 by means of three to four screws 15a. In such a structure, deformation is caused between the vicinity of the portions of the rotatable polygonal mirror member which are fixed by the screws and the portions which are not fixed by the screws, and this in turn may cause the surface fall. The fixing device shown in FIG. 2b is such that to prevent the occurrence of the surface fall in the structure shown in FIG. 2a, rigid ring plates 20 are interposed between the heads of the fixing screws 15a and the rotatable polygonal mirror member 1, and again in this case, the surface fall similar to that in the case of FIG. 2a occurs more or less. Also, in the case of FIG. 2a, the irregularity of the fastening torques of the screws directly affects the surface fall. The fixing device shown in FIG. 2c is such that the boss portion 1b of the rotatable polygonal mirror member 1 is threaded and the polygonal mirror member is fixed to the rotary shaft 2 by a set screw 15, and again in this case, the surface fall occurs due to the back-lash with respect to the rotary shaft 2. Also, where a form similar to that shown in FIG. 2c is adopted and the rotatable polygonal mirror member 1 is fixed to the rotary shaft 2 not by screws but by an adhesive agent, the same result occurs due to the irregularity or the like of the adhesive agent.
Where the rotatable polygonal mirror member is thus fixed by means of screws or the like, the unbalance during rotation becomes particularly great and therefore, adjustment of such unbalance becomes necessary. However, the unbalance is greatly varied simply by turning the screws several times and therefore, the adjustment of the unbalance is highly difficult, and this leads to irregularity of products and increased manufacturing cost. In addition, the apparatus becomes bulky to achieve the stability of rotation and accordingly becomes heavy in weight.
Thus, compact, light-weight light beam scanning apparatuses have been desired and moreover, apparatuses which are high in scanning accuracy and capable of effecting beam scanning at high speeds have been desired eagerly from the viewpoint of enhanced recording speed as well.
In the construction of the rotating device as described above, a part of the bearing member 7 is exposed to the rotatable polygonal mirror member 1 side and oil mist 21 created from the lubricant in the bearing 7 with rotation scatters outwardly during rotation of the shaft, whereby the oil mist adheres to the mirror surfaces of the rotatable polygonal mirror member 1, and this has resulted in the reduced reflection factor of the polygonal mirror member. To prevent this, sealing such as magnetic seal or labyrinth is provided near the bearing member, but the effect thereof is not sufficient when viewed from the viewpoint of long-time use and in the apparatuses of the high-speed rotation type, accurate scanning of the beam is difficult and image vibration is liable to occur.
In the past, there has been no light beam scanning apparatus which can prevent the reduction in scanning accuracy resulting from such cause and moreover, there has been no light beam scanning apparatus which can be made compact.